The present invention relates to a method of making a welded can body formed by electric-resistance mash-seam welding and to be used for a food can, a carbonate drinks can or the like, particularly the can body having a weld of improved appearance which can be adequately covered or mended with a protecting material.
There is known a method of making a welded can body by using a so-called Soudronic welding machine of the type disclosed in the Japanese Examined Patent Publication No. 26213/1979 corresponding to U.S. Pat. No. 4160 892. In this method, the opposite marginal edge portions of a metallic blank, such as of tinplate are overlapped to form a can body preform with an overlapped portion, and then the overlapped portion is mash-seam welded to form a welded can body, by using a pair of wire electrodes, each having a flat surface abutting on the surface of the overlapped portion.
The mash seam weld thus formed is normally 1.3 to 1.7 times as thick as the blank sheet, and a step portion (refer to 22 in FIG. 2) with a relatively sharp slope is produced beside the weld. Further, when welding force or welding current is increased to raise weld strength, so-called "extruded iron" (refer to 23 in FIG. 2 and 33 in FIG. 8) with an irregular configuration and extending outwardly in the circumferential direction tends to be produced, or splashes (splashes of molten iron, refer to 34 in FIG. 8) tend to attach to the vicinity of the extruded iron.
Accordingly, there arises a problem that, when the areas having the extruded iron with an irregular configuration or the splashes are mended or coated by the application of lacquer or the like to prevent the interior surface of the weld and its vicinity from corroding by contents, non-coated portions or extremely thinly coated portion tend to be generated, thus making it difficult to attain satisfactory effect of the mending.
When a welded can body is used with its exterior surface being exposed to a bad corrosive environment, the same problem as above occurs on the exterior surface of the weld.
When contents are dry articles, oily liquid, oily aerosol or the like, the problem of interior surface corrosion hardly occurs. Even in this case, however, there arises another problem that extruded iron with an irregular configuration and splashes produced on the exterior surface beside the weld tend to injure the appearance, reducing the commercial value of the can body. Also, when a can is opened, the interior surface of the weld is readily visible, tending to cause the same problem as above.
Furthermore, a welded can body with satisfactory weld strength has been practically unable to be manufactured by the conventional welding method described above from a can body preform made of a surface treated steel sheet having a surface treatment layer with relatively high electric insulation, such as tinfree steel, without removing the surface treatment layer on the marginal edge portions to be welded of the blank.